1. Technical Field
The present invention relates to a piezoelectric transducer, a piezoelectric actuator, and a portable device.
2. Related Art
In addition to reducing the size and thickness of parts used in small, portable electronic devices such as wristwatches, it is also conventionally desirable to reduce power consumption in order to extend battery life. This has led to piezoelectric actuators being used instead of stepping motors as the drive device in timepieces, for example, because of their small size, thinness, and superior efficiency converting electrical energy to mechanical energy. See, for example, Japanese Patent 3832260 (FIG. 6 and paragraphs 0008 and 0009), Japanese Unexamined Patent Appl. Pubs. JP-A-H08-114408 (FIG. 7), JP-A-H06-104503 (FIG. 1), JP-A-2004-254417 (FIG. 2 and FIG. 7), and WIPO Pamphlet 96/14687 (page 14, lines 15-24, and FIG. 16).
As shown in Japanese Patent 3832260, the piezoelectric actuator used in this type of portable device has a flat, substantially rectangular piezoelectric transducer that has a reinforcing member laminated with a rectangular piezoelectric element, uses the piezoelectric transversal effect produced by applying an electrical field in the thickness direction of the piezoelectric element to cause the piezoelectric transducer to vibrate in the direction (in-plane direction) along the surface perpendicular to the direction of the applied field, and drives a rotor or other driven body by transferring this vibration to the driven body.
Note that the “in-plane direction” as used herein is the direction of the plane that is perpendicular to the direction of the field applied to the piezoelectric element. This in-plane direction is a set of plural vectors perpendicular to the direction of the field applied to the piezoelectric element. The piezoelectric transducer of the present invention vibrates in this in-plane direction. In addition, any direction (any direction deviating from this in-plane direction) intersecting this in-plane direction is referred to as an “out-of-plane direction.”
The reinforcing member that is laminated to the piezoelectric element in the piezoelectric transducer taught in Japanese Patent 3832260 has a stationary part, a pair of movable parts, and a pair of arm parts. The stationary part is fixed to a device-side support member, for example, and is disposed near the plane center of the piezoelectric element. The pair of movable parts are disposed along the short sides of the piezoelectric element, and the pair of arms connect the stationary part with the movable parts. A protruding tip is rendered to each of the movable parts, and one of the protruding tips is set in contact with the rotor (driven body). The reinforcing member is shaped this way in order to increase the amplitude of and stabilize sinusoidal vibration, which is typically difficult to control. The area of the reinforcing member is smaller than the area of the piezoelectric element while leaving a place to attach the reinforcing member to the device and a place for contact with the driven body, and the rigidity of the reinforcing member is reduced.
Japanese Unexamined Patent Appl. Pub. JP-A-H08-114408 teaches forming an opening in the reinforcing member for greater flexibility and reduces the rigidity of the reinforcing member by reducing the thickness of the reinforcing member in places.
JP-A-H06-104503 teaches reducing the rigidity of the reinforcing member by forming a slit in the rectangular reinforcing member so that the flexural rigidity across the width is greater than the flexural rigidity along the length.
Similarly, JP-A-2004-254417 teaches reducing the rigidity of the reinforcing member by making the reinforcing member small enough to contact only a part of the piezoelectric element.
WIPO Pamphlet 96/14687 teaches reducing the rigidity of the reinforcing member by making the reinforcing member from a material with good flexibility.
Demand for increasingly smaller and thinner electronic devices continues unabated, and growing use of piezoelectric actuators has also created demand for piezoelectric actuators that can drive heavier loads, piezoelectric actuators that can continuously drive a driven body, and piezoelectric actuators that can achieve a high drive speed. To achieve this requires increasing the amplitude of the piezoelectric transducer and improving the drive efficiency.
Reducing the size and thickness of the device and increasing the amplitude of the piezoelectric transducer are usually mutually exclusive. The applied voltage drops if the size of the power supply (battery) is reduced, the amplitude therefore also decreases, and it is difficult to achieve the desired drive characteristic. Conversely, increasing the applied voltage to increase the amplitude typically requires a larger power supply. Note that as the size of the power supply decreases and the applied voltage decreases, vibration of the piezoelectric element decreases and the drive efficiency relative to the input power drops, and driving the driven body may not be possible. It is therefore necessary to greatly increase drive efficiency.
Simply reducing the rigidity of the reinforcing member by using a reinforcing member with less area than the piezoelectric element as taught in Japanese Patent 3832260, JP-A-H08-114408, JP-A-H06-104503, and JP-A-2004-254417, or reducing the rigidity of the reinforcing member by making the reinforcing member from a material with good flexibility as taught in WIPO Pamphlet 96/14687, does not mean that the amplitude can be reliably increased and is still far from achieving the required drive efficiency. This is because the relationship between the rigidity of the reinforcing member and the vibration energy density in different parts of the piezoelectric transducer in specific vibration modes is not considered. Unless the significance of the different shapes of low rigidity reinforcing members is better understood, reliably increasing amplitude in a specific vibration mode and achieving high drive efficiency will not be possible.
Japanese Patent 3832260, JP-A-H08-114408, JP-A-H06-104503, and JP-A-2004-254417 teach reducing the rigidity of the reinforcing member by changing the shape of the reinforcing member. Contact state between the piezoelectric element and the reinforcing member at the node and antinode of vibration in Japanese Patent 3832260, JP-A-H08-114408, JP-A-H06-104503, and JP-A-2004-254417 is shown in Table 1.
TABLE 1JP 3832260JP-A-H08-114408JP-A-H06-104503JP-A-2004-254417Node of longitudinalcontactcontactcontactcontactvibrationAntinode of sinusoidalno contactcontactcontactno contactvibration
Because the reinforcing member and the piezoelectric element in Japanese Patent 3832260, JP-A-H08-114408, JP-A-H06-104503, and JP-A-2004-254417 are in contact at the node of longitudinal vibration where the strain of longitudinal vibration is greatest, vibration of the piezoelectric transducer is impeded and vibration efficiency drops.
Vibration of the piezoelectric transducer is also impeded and vibration efficiency drops with the devices taught in JP-A-H08-114408 and JP-A-H06-104503 because the reinforcing member and the piezoelectric element are in contact at the antinode of the sinusoidal vibration where the strain of sinusoidal vibration is greatest. Note that in JP-A-H08-114408 the electrode functions as a reinforcing member because there is no reinforcing member per se.
In JP-A-2004-254417 the reinforcing member is present and is in contact with the piezoelectric element in the center of the width at the node of longitudinal vibration.
The reinforcing member and the piezoelectric element do not touch at the antinode of sinusoidal vibration in Japanese Patent 3832260 and JP-A-2004-254417. However, because the shape of the part (this part is referred to as the arm parts in Japanese Patent 3832260) of the reinforcing member extending substantially lengthwise through the widthwise center part of the rectangular piezoelectric element is not particularly considered and this part is narrow in width, the reinforcing member may not be able to limit displacement in any direction other than the direction (in-plane direction) along the plane perpendicular to the direction of the field applied to the piezoelectric element. As a result, it may not even be possible to achieve the desired effect of increased amplitude.
More specifically, because displacement in a direction (out-of-plane direction) other than the direction (in-plane direction) along the plane perpendicular to the direction of the field applied to the piezoelectric element occurs, the amplitude of vibrations in the direction along the plane perpendicular to the direction of the field applied to the piezoelectric element is attenuated.
The tendency for displacement in a direction (out-of-plane direction) other than the in-plane direction was analyzed using a piezoelectric transducer 100 (FIG. 33) that has a reinforcing member (FIG. 32) with a smaller area than the piezoelectric element, substantially the same as the reinforcing member taught in Japanese Patent 3832260 and JP-A-2004-254417. FIG. 34 and FIG. 35 show the results of this analysis.
As shown in FIG. 32, the reinforcing member 101 has a rectangular outside shape (indicated by the double-dot dash line) with four portions of equal length and width removed from the reinforcing member 101 as shown in FIG. 32, rendering void portions 101A. The piezoelectric transducer 100 shown in FIG. 33 has this reinforcing member 101 and piezoelectric elements 102 that are bonded to the front and back surfaces of the reinforcing member 101. A non-contacting portion 103 where there is no contact between the reinforcing member 101 and the piezoelectric elements 102 is rendered in the areas where the void portions 101A are formed in the reinforcing member 101 (FIG. 32), and these non-contacting portions 103 are rendered at the same positions as the void portions 101A.
Fives electrodes configured identically to the five electrodes 231 to 235 shown in FIG. 3 are disposed to the piezoelectric elements 102. FIG. 34 shows the vibration of the piezoelectric transducer 100 when voltage is applied to the electrodes of the piezoelectric elements 102 of the piezoelectric transducer 100 that correspond to electrodes 232, 233, and 234 in FIG. 3, and voltage is not applied to the electrodes corresponding to electrodes 231 and 235 in FIG. 3. Not applying voltage to some of the electrodes on the piezoelectric elements 102 produces an imbalance in the longitudinal expansion and contraction of the piezoelectric elements 102, and thus induces sinusoidal vibration in a direction perpendicular to the longitudinal direction of the piezoelectric elements 102. Note that FIG. 34 shows what happens when a rotor or other driven body does not contact the piezoelectric transducer 100 and the piezoelectric transducer 100 vibrates in a no-load state.
FIG. 34 shows the result of a computer simulation of the vibration state of the piezoelectric transducer in FIG. 33. FIG. 35 shows only the piezoelectric element 102 of the piezoelectric transducer 100 in FIG. 34 when vibrating. FIG. 34 and FIG. 35 exaggerate the actual displacement of the piezoelectric elements 102 and reinforcing member 101, but it will be obvious from FIG. 34 and FIG. 35 that vibration on the z-axis in the non-contacting portions 103 (FIG. 34) where there is no contact between the reinforcing member 101 and the piezoelectric elements 102 is greater than the parts where the reinforcing member 101 and the piezoelectric elements 102 touch. As a result, displacement in a direction other than the in-plane direction (i.e., the out-of-plane direction) results if the size of the voids is increased indiscriminately.